The need to reduce the emission of harmful byproducts and the ever-increasing pressures to increase fuel economy are ongoing challenges with internal combustion engines. NOx, which consists of the oxides of nitrogen, constitute a major component of these byproducts.
Lean-burn engines are designed to operate with a very lean air-fuel ratio during light load conditions, in contrast to traditional gasoline engines which are designed to run at a chemically correct (stoichiometric) air fuel ratio of about 14.7:1—which is optimal for three-way catalyst performance. Lean burn engines mix excess air with the fuel during light load conditions when full power is not needed, resulting in better fuel economy. The air/fuel ratio in lean burn port-fuel-injected gasoline engines can be as high as 22:1, and for direct-injected (DI) gasoline engines operating under stratified-charge conditions, the air/fuel ratio can be as high as 30:1. When full power is needed during heavy load conditions, such as during acceleration or hill climbing, a lean burn engine reverts to a stoichiometric (14.7:1) ratio or richer.
To illustrate the tradeoffs, if engine efficiency is increased by increasing the air to fuel ratio to a lean condition, carbon monoxide and particulate emissions are reduced. However, the three-way catalyst that is used for emission control on current stoichiometric engines has inadequate NOx conversion efficiency in a lean atmosphere. Conversely, if the combustion process is modified to reduce nitrogen oxide production, i.e., by running the engine under rich-burn (excess fuel) conditions, particulates, carbon monoxide, and hydrocarbon emissions are increased and fuel efficiency is drastically reduced.
Selective Catalytic Reduction (SCR) is the process of selectively reducing NOx across a catalyst under lean conditions. Selective catalytic reduction with a reductant, such as urea or NH3, is an effective technology for treating the NOx emissions from diesel engines. The SCR catalyst uses base metals to promote the reaction between NOx and NH3 in order to produce N2 and H2O under lean conditions. SCR catalysts can reduce NOx over a broad range of temperature, and since they contain no precious metals, they are a cost-effective approach for diesel NOx control. One issue with SCR is the need to carry an extra reductant on board the vehicle. This reductant is usually urea since there are safety issues with the handling of NH3. The urea breaks down into NH3 in the exhaust system, and the NH3 reacts with NOx over the SCR to form N2.
Selective Catalytic Reduction (SCR) with urea or NH3 injection is also a leading candidate for NOx control on lean-burn gasoline engines, either lean-burn port-fuel-injected (PFI) or stratified-charge DI engines. However, gasoline engines normally operate at stoichiometric or rich A/F ratios during high load operation, and the resulting hot rich exhaust or hot stoichiometric exhaust is detrimental to the durability of the SCR catalyst, with the result that its NOx conversion capability is lowered.
Accordingly there exists a need to protect the SCR catalyst during high temperature rich or stoichiometric operation, so that high NOx conversion efficiency can be maintained when the exhaust temperatures are lowered and the air/fuel ratio returns to a lean condition.